This application claims the priority of German patent document 197 00 738.4, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for regulating the quantity of fuel injected by fuel injectors in an internal-combustion engine.
The operating behavior of an internal-combustion engine depends significantly on the combustion conditions in the individual cylinders. Efficient operation requires identical optimal fuel/air mixture conditions in each cylinder. In fuel-injecting internal-combustion engines, it is therefore necessary (if possible) that the injectors inject the respective identical fuel quantity during each injection.
Injection systems for internal-combustion engines are known in which the injectors are fluidically connected with a common pressure line which carries fuel under a static pressure. The injectors can be opened up for injection at different points of time. A fuel pump redelivers the fuel quantities taken from the pressure line and generates a certain static nominal pressure which is applied to each pressure line and therefore to the respective injectors, at least before each injection. In such injection systems, called common-rail systems, the injection quantity of the injectors depends not only on the static pressure in the common pressure line and the injection time between the opening and the closing of the respective injector during the injection, but also depends to a large degree on the geometrical dimensions and the condition of the respective injectors. Although the injection period is the same, varying injection quantities of different injectors may be caused, for example, by tolerances of the injection nozzles or by a locally varying vibration behavior of the fuel in the environment of an injector. However, the main cause of varying injection quantities is the dirtying of the injectors by deposits as the service life of the internal-combustion engine advances, such that the fuel flow rate of a dirty injector is reduced.
In order to ensure identical injection quantities of all injectors, British Patent Document GB-2277386A suggests a process for detecting the condition of the injectors in the injection system during the operation of the internal-combustion engine. When an injection quantity is diagnosed which deviates from the desired value, the injection time of the injector is changed in the subsequent injections, so that the desired fuel quantity is injected in each case. The reduced injection quantity of a dirty injector is therefore compensated by a corresponding extension of the injection period. A regulator unit supplies to each injector an individual control signal for opening and closing with an injection period corresponding to the injection quantity. The pressure line is supplied with fuel by a fuel pump which generates a largely constant fuel pressure in the pressure line. Pressure losses in the pressure line caused by injections are immediately compensated by the fuel pump. In order to limit the fuel pressure to a defined nominal pressure during the delivery of the fuel, when the nominal pressure is exceeded, a pressure regulator opens up a return flow line to the fuel tank.
In order to generate measuring signals which can be supplied to the regulator unit and can be used as a basis for regulating the flow rates (and thus the injection quantities of the individual injectors), a pressure sensor is arranged in the pressure line. The pressure is regulated by changing the individual control signals of the injectors.
The pressure sensor detects reflection pressure waves which are caused by the operation of individual injectors during each injection and, from the time sequence of the reflection waves in the pressure line, generates a correlating electric signal which is supplied to a processor unit for analysis. Simultaneously, the processor unit receives a timer signal from the regulator unit in order to synchronize the detected reflection pressure waves with the operation of the injectors, and to assign each pressure wave signal to the responsible injector.
The processor unit's analysis of the time sequence of the electric signal of the pressure sensor yields the information concerning the qualitative time sequence of the reflection pressure waves in the pressure line caused by individual injectors. The time variations of the individual reflection pressure waves or of the pertaining electric pressure sensor signals are detected by the processor unit after the release by the injectors. For this purpose a plurality of successive measuring values are collected during respective analyzing periods. During a first analyzing period before the opening of an injector, a first series of multiple measuring values is taken, while in a second analyzing period (during the injection), another series of multiple measuring values is taken, from the electric signal of the pressure sensor. From the measuring values of both series, in each case the processor unit determines one average value and generates an output signal from the difference between the two average values. By means of this output signal, information is supplied to the regulator unit concerning the flow rate and the injection quantity of the diagnosed injector. If the dirtying of an injector is discovered, the regulator unit will change the control signals of the concerned injector, based on the output signal of the processor unit.
The known process requires high expenditures during the analysis of the pressure signal for generating a regulating quantity for the regulator unit, as well as a complicated electronic system. Particularly when the process is used for regulating the injection quantities of injectors of a common-rail injection system of an internal-combustion engine, it is difficult to control disturbing influences on the pressure sensor in the pressure line. For example, different response times of the pressure sensor during injections, for injectors at varying distances from the sensor, caused by the respective propagation rate of the reflection wave or by the disturbing reflection waves of the fuel pump or of the pressure regulator in the fuel line which, in an interfering manner, are superimposed on the reflection waves of the injectors to be measured and analyzed.
It is therefore an object of the present invention to provide a simple and reliable process and apparatus for regulating the injection quantities of injectors.
This and other objects and advantages are achieved by the regulating process according to the invention, in which after the closing of an injector, in each case the pressure gauge measures the static differential pressure in the pressure line and generates the measuring signal therefrom. From the measuring signal, the regulator unit determines the pressure difference between the nominal pressure in the pressure line before the opening of the injector and the differential pressure which exists because of the injection and the connected fuel removal from the pressure line. From the pressure difference between the absolute pressure values before and after the injection (nominal pressure and differential pressure), the precise injection quantity of the diagnosed injector can be determined by multiplying the pressure difference with the quotient from the known overall volume of the pressure line and the modulus of elasticity of the fuel. If the actual injection quantity deviates from a known desired value, the regulator unit corrects the individual control signals of the respective injector by changing the injection period by an amount corresponding to the deviation from the desired value. If, for example, an excessively reduced injection quantity, (due, perhaps to dirtying of the respective injector) is determined, during the subsequent triggering of this injector, the injection period will be extended by supplying correspondingly formed control signals.
The easy determination of the actual injection quantity is based on the compressibility characteristic of the fuel. The static pressure is formed by compressing, and is equivalent to a tendency of the fuel to expand. Since the compressibility of the fuel is determined by the (constant) modulus of elasticity, which defines the slope of the linear dependence of the volume change on the pressure change, the precise volume change in the pressure line can be determined based on the modulus of elasticity and a measurement of the pressure reduction by the expansion of the compressed fuel.
Advantageously, the fuel in the pressure line is subjected to a high static pressure of at least 100 bar. Because of the high static nominal pressure, the fuel in the pressure line is compressed such that large fuel quantities in comparison to the standard volume are stored in the pressure line. Preferably, the fuel is compressed in the pressure line by a pressure of approximately 1,500 bar, whereby short injection periods can be achieved with an exact measurement of the injection quantity, utilizing the compressibility characteristic and the expansion characteristic of the fuel.
In operating intervals between completed individual injections, the fuel pump delivers fuel into the pressure line. Each such operating interval is concluded when the nominal pressure is reached, so that the fuel quantity which was removed from the pressure line during the preceding injection is restored. (The nominal pressure in the pressure line is reached in each case before the start of the next injection and the opening-up of the corresponding injector.) If the differential pressure in the pressure line formed by the injection is measured by a pressure gauge, after the closing of the injector and before the fuel pump is started, a precise measuring signal can be generated for determining the fuel quantity actually removed from the pressure line due to the injection, and can be supplied to the regulator unit. During the time periods between the operating intervals and the injections, the respective static pressure in the pressure line will be constant. Thus, the detection of the nominal pressure before an injection and the differential pressure after the injection in corresponding time periods outside the operating interval of the fuel pump, permits a precise determination of the pressure difference which occurs during an injection, and thus of the actual injection quantity.
Advantageously, the operating intervals of the fuel pump are determined by the regulator unit, and start only after the presence of a measuring signal following measurement of the differential pressure after a fuel injection. The operating intervals of the fuel pump, and thus the further pressure rise in the pressure line because of the fuel delivery, will in each case be terminated by the regulator unit when the pressure gauge detects the normal pressure has been attained in the pressure line.
Despite short injection periods, a precise detection of the injection quantity is possible during the diagnosis of the injectors since the course of the pressure which is difficult to analyze at high pressures, during an injection, is unimportant for regulating the injection quantities. The required measuring signal is generated in a simple manner and with the highest precision from the difference between the nominal pressure in the pressure line before an injection and the differential pressure it after an injection. In this case, for measuring the nominal pressure or the differential pressure, respective arbitrary points in time are available to the pressure gauge in the time periods between the operating intervals of the fuel pump and the injections. If the pressure measuring signals are generated simultaneously with the respective operation of an injector (opening, closing), an operation is possible with rapidly successive injections, with the respective regulating of the injection quantity.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.